1. Field of the Invention
This invention relates to a BGA (ball grid array) type semiconductor device which has ball-shaped conductive terminals.
2. Description of the Related Art
A CSP (chip size package) receives attention in recent years as a three-dimensional mounting technology as well as a new packaging technology. The CSP is a small package having about the same outside dimensions as those of a semiconductor die.
A BGA type semiconductor device has been known as a kind of CSP. A plurality of ball-shaped conductive terminals made of a metal such as solder is arrayed in a grid pattern on one principal surface of the BGA type semiconductor device and is electrically connected with the semiconductor die mounted on the other side of the package.
When the BGA type semiconductor device is mounted on electronic equipment, the semiconductor die and an external circuit on a printed circuit board are electrically connected by compression bonding of each of the conductive terminals to each of wiring patterns on the printed circuit board.
Such a BGA type semiconductor device has advantages in providing a large number of conductive terminals and in size reduction over other CSP type semiconductor devices such as an SOP (small outline package) and a QFP (quad flat package), which have lead pins protruding from their sides. The BGA type semiconductor device can be used, for example, as an image sensor chip for a digital camera incorporated into a mobile telephone.
FIGS. 10A and 10B show an outline structure of a conventional BGA type semiconductor device. FIG. 10A is an oblique perspective figure of a front side of the BGA type semiconductor device. FIG. 10B is an oblique perspective figure of a back side of the BGA type semiconductor device.
A semiconductor die 104 is sealed between a first glass substrate 102 and a second glass substrate 103 through epoxy resins 105a and 105b in the BGA type semiconductor device 101. A plurality of ball-shaped terminals 106 is arrayed in a grid pattern on a principal surface of the second glass substrate 103, that is, on the back side of the BGA type semiconductor device 101. The conductive terminals 106 are connected to the semiconductor die 104 through second wirings 110. The second wirings 110 are connected with aluminum wirings pulled out from inside of the semiconductor die 104, making each of the ball-shaped terminals 106 electrically connected with the semiconductor die 104.
Detailed explanation on a cross-sectional structure of the BGA type semiconductor device 101 will be given referring to FIG. 11. FIG. 11 shows a cross-sectional view of the BGA type semiconductor devices 101 divided along dicing lines into individual dice.
A first wiring 107 is provided on an insulation film 108 provided on a surface of the semiconductor die 104. The semiconductor die 104 is bonded on the first glass substrate 102 with the resin 105a. A back side of the semiconductor die 104 is bonded on the second glass substrate 103 with the resin 105b. 
One end of the first wiring 107 is connected to the second wiring 110. The second wiring 110 extends from the end of the first wiring 107 to a surface of the second glass substrate 103. And the ball-shaped conductive terminal 106 is formed on the second wiring 110 extending onto the second glass substrate 103.
The described BGA type semiconductor device 101 having a V-shaped groove VG is formed with a protection film 111 on its surface by using an organic resin before the described dicing process (FIG. 12A). For forming the protection film 111 on a surface of the second wiring 110, a method in which the organic resin is dropped from above on the back surface of the semiconductor die 104, which is placed upward, and the protection film 111 is formed by utilizing centrifugal force generated by rotating the semiconductor wafer itself, has been employed.
However, in this method, the organic resin having a thermosetting property is over-deposited and thickened more than required on a bottom of the V-shaped groove VG at a dicing line (broken line in FIG. 12A) as shown in FIG. 12A. This is because that the organic resin is pasty and of adhesive characters. Therefore, when the protection film 111 is thermally set by baking, the organic resin deposited in the V-shaped groove VG shrinks more than organic resin covering other parts of the semiconductor device 101. This causes the organic resin in the V-shaped groove VG to shrink largely, thereby warping a semiconductor wafer which is to be divided into semiconductor dice afterward (warp it in a direction shown by an arrow in FIG. 12B).
Such a warped semiconductor wafer causes a problem in the subsequent manufacturing process. Particularly, in a reflow process (high heat treatment) of conductive terminals 106 (so-called bump electrode) made of a conductive material having thermal fluidity such as solder, the semiconductor wafer can not be uniformly heated overall, thereby lowering reliability.
For example, when reflowing the conductive terminals 106 as shown in FIG. 13, the warped semiconductor wafer 124 is mounted on a stage 123 in a reflow device 120. The reflow device 120 is kept at a predetermined temperature by thermal radiation “a” emitted downward from IR (infrared) heaters 121 provided on a ceiling and thermal radiation “b” emitted upward from hot plates 122 provided under the stage 123. Arrows in FIG. 13 show a schematic view of the thermal radiation “a” and the thermal radiation “b”. Broken line circles in FIG. 13 show warped portions formed at ends of the semiconductor wafer 124. FIG. 14 is an enlarged view of one of the portions enclosed in the broken line circles.
As shown in FIG. 14, the semiconductor wafer 124 has a flat portion (hereafter, referred to as a flat portion 125), and a warped portion at an end of the semiconductor wafer 124 (hereafter, referred to as a warped portion 126). The flat portion 125 is in direct contact with the lower stage 123 (hereafter, referred to as a direct contact portion 127). On the other hand, the warped portion 126 is not in contact with the stage 123 since it is warped.
If the conductive terminals 106 are reflowed in this state, the direct contact portion 127 is heated to higher temperature than required by the hot plates 122.
On the contrary, the warped portion 126 is radiated with thermal radiation “a1” and thermal radiation “a2” from the IR heaters disposed thereabove. The thermal radiation a1 heats the end of the semiconductor wafer 124 and the thermal radiation a2 heats the inside thereof. Since there is a difference in intensity between the thermal radiation “a1” and “a2”, it is difficult to heat the semiconductor wafer 124 uniformly. Furthermore, there is another difference in intensity between thermal radiation “b1” and “b2”. The thermal radiation b1 is radiated to a lower part of the end of the semiconductor wafer 124 and the thermal radiation b2 is radiated to a lower part close to the flat portion 125 of the semiconductor wafer 124.
Therefore, a temperature difference develops between the warped portion 126 and the flat portion 125 in the semiconductor wafer 124.
Thus, the temperature difference depending on positions in the semiconductor wafer 124 makes it difficult to form the conductive terminals 106 of a uniform shape on the semiconductor wafer 124. As a result, a yield and reliability of the semiconductor device are markedly reduced.
The invention is directed to overcoming the disadvantage that the warped semiconductor wafer formed in a manufacturing process of the BGA type semiconductor device affects the reflow process of the conductive terminals.